DESCRIPTION: Exposure to noise injures the cochlea, often irreversibly. Depending on the intensity and duration of the exposure, the ear may sustain a temporary (i.e., TTS) or permanent (i.e., PTS) threshold shift. Moderate hearing losses may have devastating effects on all aspects of a person's life, including oral communication, employment opportunities and the enjoyment of some of life's greatest pleasures - the human voice, music and the sounds of nature. Several hypotheses exist on the pathogenesis of ITS and PTS, but none have been proven. No hypothesis has attempted to explain how workplace noise damages then destroys sensory cells, supporting cells and nerve fibers in the cochlea. The studies proposed here will: 1) determine how TTS and PTS are related; 2) determine which existing noise-damage hypotheses are most likely to be correct; and 3) identify factors that are associated with increased noise susceptibility. Chinchillas will be exposed binaurally for 24 hrs to a 4-kHz or a 0.5 kHz octave band of noise. The exposure level will be either 95 dB or 85 dB sound pressure level (SPL). These levels will produce a moderate TTS and sometimes a PTS. Auditory brainstem response (ABR) thresholds and distortion product otoacoustic emission (DPOAE) levels will be monitored before, and several times after exposure. The survival-fixation technique will be used to preserve an animal's two cochleas at two different times. Thus, each animal will provide two 'snapshots' of the dynamic structural changes which occur post-exposure, This technique is effective because there is excellent left-right correlation with respect to damage when both cochleas are preserved simultaneously. Problems with data interpretation resulting from inter-animal variations in noise susceptibility will be minimized. Because survival-fixation preserves the presumed in-vivo relation between the stereocilia and the tectorial membrane (TM), this project will examine alterations in the TM-stereocilia relationship at different intervals post-exposure. Hypotheses to be tested include: TTS results from pillar buckling and sagging of the reticular lamina which uncouples the stereocilia from the TM; TTS disappears when the pillars are repaired, the height of the organ of Corti is restored and the stereocilia are recoupled to the TM. Quantitative data on hair-cell and pillar losses and damage will be collected, compared between each animal's left and right cochleas and correlated with ABR threshold shifts and DPOAE changes. Our goal is to provide information on possible mechanisms of noise damage in the cochlea as a basis for developing scientifically sound strategies for reducing noise-induced hearing loss in humans.